I have a number of 3.5" hard disk drives of various sizes… and just got a unit so I can hot-swap any of these drives in/out of the system.
Does anyone know of a way to get an accurate assessment of the over-all health of a drive this way?
Seems the S.M.A.R.T. data requires special software, but I’d like something that can do a deep scan to find bad/failing sectors if any etc.
This would be for macOS Mojave
there isnt any software that I know of on OSX that does that. Your best bet is to look at the S.M.A.R.T. data off the drives.
I use DriveDX. It needs to download and install another driver when you try to access USB drives but It works fine for internal and external harddisks and SSDs to check their health.
Btw you can use DriveDX for free for 15 days.
I know it isn’t you preferred solution, but for many years I have been exercising unknown drives using SpinRite on an old PC. SpinRite reads and writes to every sector of the disk and if there is a problem it will be found.
I had found that just a few minutes after asking 
Problem is it says one drive I have is 100% fine, when in fact my NAS said it was failing.
many NAS ( specially those able to do raid 5 or more) have to choose their disks on stronger capacities.
for example my 8 disks qnap refused to use some caviar green disks although they were perfectly fine.
the NAS explores the smart values (and sou-smart) but also the kind of the disk
[quote=424955:@Markus Winter]I use DriveDX. It needs to download and install another driver when you try to access USB drives but It works fine for internal and external harddisks and SSDs to check their health.
Btw you can use DriveDX for free for 15 days.[/quote]
I just downloaded this, it’s only giving me 4 days. Also: to check external USB drives it seems to want me to uninstall a driver and install a more recent version. It does not, however, indicate where this newer version is. Supposedly, it comes with the non-AppStore version of DriveDX, but I see no evidence of that. I’m a bit unwilling to uninstall the existing S.M.A.R.T. driver just by using Terminal to remove it (“sudo rm” seems a bit drastic).
That sounds like a previous version had been installed before. AFAIK it installs and uses its own driver, and if that is out of date (eg no APFS) then of course it needs updating.
Well I tried it and installed the 0.1 driver version, and Disk Utility and DriveDX can see the SMART info from my external USB drives, which is useful (especially as I have a flaky drive).